Competing Annulene and Radialene Structures in a Single Anti-Aromatic Molecule Studied by High-Resolution Atomic Force Microscopy

According to Hückel theory, an anti-aromatic molecule possessing (4n)π-electrons becomes unstable. Although the stabilization has been demonstrated by radialene-type structures - fusing aromatic rings to anti-aromatic rings - in solution, such molecules have never been studied at a single molecular level. Here, we synthesize a cyclobutadiene derivative, dibenzo[b,h]biphenylene, by an on-surface intramolecular reaction. With a combination of high-resolution atomic force microscopy and density functional theory calculations, we found that a radialene structure significantly reduces the anti-aromaticity of the cyclobutadiene core, extracting π-electrons, while the small four-membered cyclic structure keeps a high density of the total charge.